(1) Field of the Invention
The present invention relates to a solid-state imaging device.
(2) Description of the Related Art
In recent years, demand for video cameras and digital still cameras tends to increase year by year. With increase in demand, clientele is broadened. There is a demand for a low-cost camera which can capture images in which noise caused by smear and the like are reduced, not only from a conventional clientele but also from a new clientele. In addition, there is a demand for improved resolution not only for a still image but also for a moving picture in movie cameras and some DSC cameras. Thus, development that meets customers' needs is required for a solid-state imaging device, particularly, a charge coupled device (CCD) solid-state imaging device, to be used in a camera. In order to lower the cost, it is effective to increase the number of chips per a wafer. Thus, further miniaturization of a chip is necessary.
Hereafter, a conventional CCD image sensor is described first, and a smear which occurs when miniaturizing chips is described next.
FIG. 1 shows a configuration of the conventional CCD image sensor. The CCD image sensor includes: photodiodes 901 arranged in a matrix; transfer gates 902, each of which is provided for each of the photodiodes 901; vertical CCDs 903, each of which is a charge transfer path in a vertical direction arranged on a left side of each row of the photodiodes 901; a horizontal CCD 907, each of which is a charge transfer path in a horizontal direction arranged at lower ends of the rows of the vertical CCDs 903; and an output unit 908 which outputs a voltage value corresponding to charge to outside of the CCD image sensor. The photodiode 901 converts incident light to a charge and the corresponding transfer gate 902 transfers the charge converted by photoelectric conversion to the corresponding vertical CCD 903. The vertical CCD 903 transfers the transferred charge to the horizontal CCD 907. The horizontal CCD 907 transfers the charge transferred from the vertical CCDs 903 to the output unit 908. The output unit 908 converts the charge to voltage, and outputs data including the obtained voltage value to outside of the CCD image sensor.
Next, the cause of smear is described.
FIG. 2 is a cross section diagram of a pixel of a typical CCD solid-state imaging device. In the solid-state imaging device, a photodiode unit (photoelectric conversion unit) 4 includes an n-type silicon substrate 10, a p−−-type well region 11, an n-type charge accumulation region 12, and a p++-type region 13. A vertical CCD unit 5 for transferring the signal charge obtained by a photoelectric conversion includes an n-type buried channel region 14, a p-type region 15 which is formed under the n-type buried channel region 14, a gate electrode 16 made of a polysilicon film and the like.
A transfer gate 17 unit is formed between the photodiode unit 4 and the vertical CCD unit 5, and a p+-type channel stop region 18 is formed at a position opposite to the transfer gate unit 17 with respect to the photodiode unit 4. The transfer gate unit 17 and the p+-type channel stop region 18 interpose the photodiode unit 4. A gate insulator film 19 having a lamination structure of a silicon oxide film and a silicon nitride film is formed on the n-type buried channel region 14, the transfer gate unit 17 and the p+-type channel stop region 18. A light shielding film 8 such as a tungsten film is formed on the gate electrode 16 with an interlayer insulator film 20 interposed therebetween. A reflection preventing film 21 is formed on a surface of the photodiode unit 4.
Smear occurs due to the following reason. That is, upon accumulation of charges obtained through photoelectric conversion, the charges flow into the vertical CCD unit 5 without being accumulated in the photodiode unit 4, so that a false signal is generated. This is described with reference to FIG. 2. It is considered that smear mainly occurs due to the following four causes. (i) Light transmits through the light shielding film 8, reaches the vertical CCD unit 5, and is converted into electric charges in the vertical CCD unit 5, so that smear occurs. (ii) Incident light is partially leaked from an interface between the light shielding film 8 and the gate insulator film 19, and then transmits to the vertical CCD unit 5 while multiply reflecting in the light shielding film 8, between the gate insulator film 19 and the gate electrode 16, and between the gate insulator film 19 and the light shielding film 8. Thereafter, the light is converted into charges in the vertical CCD unit 5, so that smear occurs. (iii) Charges generated through photoelectric conversion at an exterior of the photodiode unit 4 are diffused, and then reach the vertical CCD unit 5, so that smear occurs. (iv) Charges generated through photoelectric conversion at a recombination region in the p++-type region 13 on the surface of the photodiode unit 4 are transferred or diffused by a weak electric field. Thereafter, the charges reach the vertical CCD unit 5, and then are detected as false signals, so that smear occurs.
It is considered that smear occurs due to the aforementioned causes. In response to formation of a finer pixel, countermeasures against smear are constantly required. This is due to the following reasons. In the case where a pixel size is simply reduced in size, a width of the transfer gate 17 shown in FIG. 2, that is, a distance between the photodiode unit 4 and the vertical CCD unit 5 becomes short. Thus, a light transmission distance in the aforementioned case (ii) becomes short, for example. Consequently, an amount of the light absorbed upon transmission decreases, and the light is more likely to reach the vertical CCD unit 5, resulting in increase of smear. In addition, since an charge diffusion distance in the aforementioned case (iii) or (iv) becomes short, the charge is more likely to reach the vertical CCD unit 5, resulting in increase of smear. As described above, the simple reduction in pixel size causes increase of smear.
In order to suppress the increase of smear, it is effective to narrow down the width of the vertical CCD unit 5 shown in FIG. 2 (see, for example, Japanese Unexamined Patent Application Publication No. 9-149425). More specifically, the vertical CCD unit 5 is designed so that the distance between the photodiode unit 4 and the vertical CCD unit 5 does not change even if a pixel is reduced. With this, a shrinkage of propagation distance of the light to the vertical CCD unit 5 in the aforementioned case (ii), and a shrinkage of transmission distance of charge to the vertical CCD unit 5 in the aforementioned case (iii) or (iv). The increase of smear can be suppressed with this method.
However, when the width of the vertical CCD unit 5 is narrowed down, it is extremely difficult to prevent decrease of the maximum amount of charge which the vertical CCD unit 5 is capable of transferring. This is due to decrease in capacity of a capacitor which is formed, between the gate electrode 16 in the upper part of the vertical CCD unit 5 and the n-type buried channel region 14 in the lower part of the vertical CCD unit 5, caused by a decrease in a charge transfer area. Thus, an uneven saturation caused by the vertical CCD unit 5 is likely to occur, and a problem such as a decrease in dynamic range occurs.
As a countermeasure of the problem, instead of the so-called progressive method in which one frame includes one field, and the charge generated in one frame is transferred at one time, a method in which one frame includes multiple fields, and the charge generated in one frame is transferred in multiple times is used. With this method, uneven saturation and the like can be prevented even if the width of the vertical CCD unit 5 is narrowed down.
However, when transferring the charge generated in one frame in multiple fields, a problem such that temporal resolution degrades occurs. For example, when a frame includes two fields, a temporal difference in the amount of the first charge and the second charge occurs for the same pixel. This is due to a difference in time when the first charge is generated in the first field and when the second charge is generated in the second field. With this, malfunctions such that image becomes out of focus when imaging a moving object, and a blurred image is generated occur.